The invention relates generally to disk storage systems having a disk cache, and in particular, to such a system in which there is a requirement for maintaining a minimum read rate in order to satisfy an application requirement.
In a typical high speed disk array system, such as that manufactured by EMC Corporation under the name Symmetrix, the disk array system is a cached disk array system which receives, from connected hosts, input/output (I/O) requests through an interconnection channel. In an open-systems environment, a typical channel is a SCSI channel and the I/O requests are composed of read operations and write operations. In the cached disk array, the write operations can all be handled by cache, thus allowing 100% of the write operations to be immediately written to cache and to be destaged later, that is, written from cache to disk. This allows for a very high rate for the write operation. On the other hand, read operations will be serviced either from cache, resulting in a high read response and designated a read "hit," or from the disk which imposes a slower rate of response since a mechanical system is interposed into the read process. This latter operation is designated a read "miss."
When the write rate is high, the method uses more and more of the disk cache to hold the data to be written (the pending write data) before it is destaged to disk. Accordingly, since cache memory for write operations is expended at the expense of the cache area used for read operations, less and less cache memory becomes available for the read operations as the pending write data builds up. The write operations also consume processing cycles of the control unit which also has to handle the read requests.
Typically, this may not be a problem. However in instances where the system is required to maintain a minimum rate of read operations, for example when the stored information on the disk represents digitized audio or digitized video that is being heard or seen in real time, a reduction in the read rate may interrupt the required continuous stream of audio or video information. Accordingly, such processes require at least a predetermined minimum read rate to insure uninterrupted decoding of the stored audio and/or video data.